The present invention relates generally to screen printing apparatus and methods, and more particularly provides a uniquely configured emulsion exposure unit, vacuum frame, layout/stripping, printing table and air filtration system which, with the exception of emulsion rinsing equipment, very compactly combines in a single structure all of the equipment and work space necessary to carry out, on a relatively large commercial scale, an indirect screen preparation and screen printing process.
The process of "screen printing" is conventionally utilized to print advertising indicia, artwork and the like on paper stock, T-shirts, caps, and a variety of other products. This process is typically carried out by providing a "positive" sheet of plastic film having the desired indicia thereon. A transfer emulsion sheet, consisting of an acetate backing sheet having a suitable layer of light sensitive emulsion coating deposited on one side thereof, is placed on the positive film sheet and the two sheets are firmly held together by a vacuum force while they are exposed to a suitable exposure light source. During the exposure process, the indicia area on the film positive sheet masks a corresponding area of the emulsion film, leaving a non-exposed area of the emulsion film which corresponds to the indicia on the film positive sheet.
After this exposure process is completed, the transfer sheet is rinsed with water to wash away the non-exposed area of its emulsion film. The transfer sheet is then placed against the bottom side of a framed screen element and suitably adhered thereto. When the remaining emulsion has dried on the screen, the acetate type backing portion of the transfer sheet is peeled away leaving the emulsion adhered to the screen. The adhered emulsion has various openings therein which correspond to the indicia originally on the film positive sheet. The finished printing screen is then checked for emulsion defects, such as pin holes and the like, by viewing over the lit top surface through the screen. Any defects in the adhered emulsion can be repaired by applying a suitable blocking material thereto.
When the assembled printing screen is ready for use, the item to be imprinted (such as a sheet of paper stock) is placed below the screen on the glass top table surface or suitable platen in appropriate alignment with the emulsion thereon. Ink is then forced downwardly through the emulsion openings in the screen onto the article to form the desired advertising indicia or artwork thereon. Typically, this inking process is carried out by using a "squeege" element which is drawn across the upper surface of the screen to flow ink downwardly through the emulsion openings therein and onto the imprintable item.
Even on a relatively small commercial scale, this type of screen printing process conventionally has required a relatively large and expensive assortment of individual screen printing equipment components which occupy a significant amount of floor space. Specifically, as conventionally carried out, this type of screen printing process requires, among other separate elements, a screen printing surface (such as a printing table) to carry out the actual imprintation portion of the process, a separate light exposure unit to effect the initial exposure of the emulsion film, a vacuum system to hold the film positive element and the transfer emulsion sheet together during the exposure portion of the process, separate screen washing apparatus for rinsing away the non-exposed area of the emulsion film, and a separate light table for use in the screen "checking" portion of the printing process and for stripping and layout purposes.
As evidenced in U.S. Pat. No. 3,964,385 to Knight, at least one attempt has been made to provide less expensive and more compactly arranged screen printing apparatus. The Knight device is designed for use in the direct screen printing process (the forerunner of the modern "indirect" method) and comprises a box constructed with one side having a removable glass sheet on which may be stacked an opaque film positive sheet, a screen with a delayed photosensitive emulsion, a foam pad or sheet, a rigid backup plate, and a hold down strap removably connectable to the box to hold these stacked elements in contact with one another so that the screen may be exposed to a flood light within the box for a time sufficient to suitably expose the photosensitive emulsion.
After the exposure portion of the process is completed, the hold down strap is removed and the stacked elements are disassembled. The screen is thereafter washed to develop the printing image and is placed inside the box to dry. After the finish screen is dried, the box is turned on its side to horizontally position an exterior vacuum plate side portion thereof. The screen is then pivotally connected to the box so that it overlies the vacuum plate. An imprintable item is then positioned on the horizontal vacuum plate beneath the pivotally mounted screen and a blower within the box is energized to draw ambient air inwardly through the vacuum plate to hold the imprintable item thereon. Ink is then applied to the screen and forced therethrough onto the imprintable item by moving a squeege member of the like across the screen.
The Knight device, though compact and portable, has a variety of limitations and disadvantages which render it generally unsuitable for use in the modern indirect screen printing process. For example, as stated in the Knight specification, the device is suitable for screen printing only on a "small scale". Because of the need to pivot the box from its screen-forming position to its printing position, the Knight device, if constructed in the much larger size necessary to imprint much larger items on a high volume commercial scale, would be highly unwieldy and difficult to use. Additionally, on even a relatively small commercial production scale, the use of the Knight device would provide an undesirably slow overall printing process due to the need to assemble, disassemble and reassemble various of its components during the overall screen formation and printing process. Additionally, because of the pressure hold down system used to hold the film positive sheet against the emulsion portion of the screen, the "fine line" printing capabilities required in the modern indirect screen printing process could not easily be achieved, if at all. The Knight device would thus be unacceptable from a printing quality standpoint in a great many commercial and artistic printing projects.
It can be seen from the foregoing that it would be highly desirable to provide more compact and relatively inexpensive screen printing apparatus which is better suited to the high quality and volume demands associated with commercial scale screen printing activities. It is accordingly an object of the present invention to provide such improved screen printing apparatus and methods associated therewith.